(1) Field of the Invention
The present application relates to a positive temperature coefficient (PTC) composition and an over-current protection device containing the same, and more particularly to a PTC composition having low volume resistivity and an over-current protection device containing the same.
(2) Description of the Related Art
Because the resistance of conductive composite materials having PTC characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the crystalline polymer of the PTC conductive composite material will melt and expand to sever a lot of conductive paths and therefore the resistance instantaneously increases to a high resistance state (i.e., trip) to decrease the current.
The PTC conductive composite material usually comprises crystalline polymer and conductive filler. The conductive filler is dispersed in the crystalline polymer. The crystalline polymer may comprise polyolefin such as polyethylene, and the conductive filler usually comprise carbon black. In recent years, the conductive filler tends to the use of conductive ceramic, e.g., tungsten carbide, in place of the carbon black to attain low resistivity.
However, impurity may be generated in sintering process of tungsten carbide, resulting in high or unstable resistivity. As a result, the PTC conductive composite material using tungsten carbide is difficult to achieve ideally low resistivity.